This invention relates to videotape recorder and playback devices (VTR's) and, more particularly, to apparatus for correcting the so-called "velocity" error component of a television signal played back on a VTR on a continuous, every line basis. Heretofore in multiple head VTR's wherein each video track is a portion of a field (known as "segmented" scan VTR's), no complete velocity compensation corrections were made to the television line adjacent a head switch.
The present invention is one portion of a VTR, portions of which are disclosed in other copending applications assigned to the present assignee, all of which are incorporated herewith by reference: Ser. No. Title Filed Inventor(s) __________________________________________________________________________ 285,919 SYNCHRONIZING PULSE 9-1-72 Bert H. Dann and PROCESSOR FOR A Nikola Vidovic VIDEO TAPE RECORDER 285,922 HELICAL SCAN WIDEBAND 9-1-72 Barrett E. Guisinger TAPE RECORDER APPAR- ATUS AND METHOD 285,924 FM PULSE AVERAGING 9-1-72 Bert H. Dann DEMODULATOR 285,917 VIDEO TAPE RECORDER 9-1-72 Kenneth Louth METHOD AND APPARATUS 314,734 TECHNIQUE FOR DETECT- 12-13-72 Nikola Vidovic ING LONG DURATION PULSES FROM A TRAIN OF SHORT DURATION PULSES 320,092 PASSIVE PEAK 1-2-73 Nikola Vidovic DETECTOR AND LEVEL SHIFTER 354,227 MAGNETIC TAPE 4-25-73 Frank S.C. Mo and TRANSPORT SYSTEM Vernon Natwick 355,220 VIDEOTAPE RECORDER 4-27-73 Barrett E. Guisinger AND REPRODUCER and Bert H. Dann VELOCITY COMPENSA- TOR APPARATUS 355,758 VIDEO PROCESSING 4-30-73 Nikola Vidovic CIRCUIT 355,781 PHASE SHIFTER 4-30-73 Nikola Vidovic APPARATUS 356,380 VIDEO TAPE RECORDER 5-2-73 Kenneth Louth METHOD AND APPARATUS 356,839 VIDEO TAPE RECORDER 5-3-73 Bert H. Dann EDITING APPARATUS 439,093 VIDEOTAPE RECORDER 2-4-74 Bert H. Dann and AND REPRODUCER Barrett E. Guisinger VELOCITY COMPENSATOR APPARATUS __________________________________________________________________________
In the videotape recorder (VTR) art the requirements of broadcast quality color television signal recording and reproduction has placed extremely stringent tolerances on the time base stability of the VTR. Modern color VTR's intended for broadcast use include several sub-systems which contribute to a relatively stable reproduced television signal: capstan and head drum servo systems for resolving large phase errors between the VTR signal and a reference signal and electronic circuits for resolving the smaller remaining phase errors left uncorrected by the servo systems. Typical VTR's employ two controllable variable delay lines in series: the first, a portion of a sub-system generally referred to as the monochrome time base corrector, is controlled by an error signal derived by comparing the phase of off-tape horizontal sync signals to reference horizontal sync signals; the second, a portion of a sub-system generally referred to as the chrominance (or color) time base corrector, provides a vernier correction and is controlled by a phase comparison between the off-tape chrominance sub-carrier (color burst) and the reference chrominance sub-carrier.
The effect of the monochrome and chrominance time base correctors is to produce a phase change in the VTR off-tape television signal so as to bring it in "precise" phase coincidence (within several nano-seconds) with the reference sync and burst signals. The phase corrections are made to the VTR off-tape television signal at the start of each horizontal line and the same correction is maintained by the time base correctors throughout the line.
It was discovered, however, that the correcting of the VTR off-tape signal at each horizontal line is not adequate. Although the VTR signal is in phase with the reference at the start of a line, the VTR off-tape signal phase departs from the reference phase through the horizontal line time so that by the end of the horizontal line there is an unacceptable phase error. Fortunately, the progressive phase error through the horizontal line is essentially linear. Thus, a further time base error correction is provided, known as "velocity" compensation, so named because it was believed to result from variations in head drum velocity due to mechanical offsets, etc. It appears that the so-called "velocity" error may also be a result of waves set up in the tape when struck by the rapidly moving video heads. Nevertheless, the exact cause of "velocity" error is of no importance to the present invention.
Because the "velocity" error is essentially a straight line error, the standard solution is to add a ramp having the proper slope to the voltage in the color time base corrector which controls the vernier delay time. Thus, the delay line is changed progressively throughout each horizontal line and the last significant phase error is compensated. Discussions of time base correction in VTR's is found in Magnetic Recording by Charles E. Lowman, McGraw-Hill, 1972; "A Short History of Television Recording: Part II" by Albert Abramson, Journal of the SMPTE, March, 1973, pp. 188-198; "A New Technique for Time-Base Stabilization of Video Recorders" by Charles H. Coleman, IEEE Transactions on Broadcasting, March, 1971, pp 29-36; U.S. Pat. Nos. 3,213,192 (Jensen), 3,428,745 (Coleman et al.); and 3,504,111 (Sumida et al.).
Prior art velocity compensators are classifiable into two groups: storage systems where the corrections are not made to the actual line in which the velocity error is measured and oneline delay systems in which the corrections are made to the actual line in which the error is measured. Both approaches suffer from the problem that no full correction is made for one line adjacent a head switch. In the type disclosed in the Coleman et al. U.S. Pat. No. 3,428,745 the VTR off-tape horizontal sync pulses are compared to reference horizontal sync pulses and the phase error between consecutive horizontal lines is stored for one head wheel rotation period. This approach assumes that the same or nearly the same velocity error occurs when the head wheel subsequently rotates to the same position. Storage means are required for the number of horizontal lines covered by a head wheel rotation (64 in the case of the "quadruplex" recorder). In such a system, the line following a head switch is not fully corrected.
In another type of velocity compensator as disclosed in the Coleman IEEE Transactions article and the Sumida et al. U.S. Pat. No. 3,504,111, a one horizontal line delay is placed in the VTR off-tape signal path and the phase of the color burst (chrominance subcarrier) before and after the delay is compared. This approach has the advantage of avoiding storage of the error measurements since the correction may be immediately applied to the one line delayed signal. In such a system, the line before a head switch is not fully corrected.
In the aforementioned prior art systems, the line before or after a head switch is uncorrected because the corrections are based on line-to-line comparisons from the same video head. Typically, the same correction is stored and used as for the previous or subsequent line. While providing a reasonably close correction, nevertheless, such errors are not fully corrected and become visible when multi-generation copies are made.
In accordance with the teachings of the present invention, every line velocity error correction is achieved by providing a mechanical overlap on the played back video tape so that two adjacent video heads simultaneously reproduce at least one clean horizontal sync pulse or horizontal sync pulse and color burst. In addition to the normal high quality main video processing channel which is switched among the heads in sequence, auxiliary low quality channels are continuously connected to each video head and velocity error detection circuitry operates in connection with the low quality channels. The low quality channels need only provide a reasonably noise-free color burst, or sync edges if used instead of the color burst, for the velocity error detection. The same head switching sequence, properly delayed, can be used to select the proper low quality channel. Due to the overlap, the line adjacent the head switch has the correct velocity error based on information not available in the main video channel. Although for the purposes of a complete disclosure the invention is described in connection with a video tape recorder as described in said U.S. Ser. No. 285,922 application and in connection with the velocity error detector of said U.S. Ser. No. 439,093 application, it is to be understood that the invention applies to all types of helical scan and quadruplex VTR's using multiple heads, and further, that the invention applies to all types of velocity error detection techniques other than that of said U.S. Ser. No. 439,093 application. For example, velocity error detectors using horizontal sync as references. Nor is the invention limited to one-line delay systems. It may be used with the aforementioned storage type system.
These and other advantages of the present invention will be better understood as this specification and drawings are read and understood.